The present invention relates generally to a short distance coupling environment, and more particularly to latency reduction in a short distance coupling environment.
For short distance coupling, like many other I/O protocols, it needs to be ensured, from a high level perspective, that a transaction is only signaled to be completed after all payloads have arrived. To accomplish this, ordering mechanisms are applied by the memory subsystem and the coupling message hardware side. For a coupling device (for example, an integrated cluster bus (ICB)), a short distance coupling protocol, two implementations for accomplishing this goal in current designs are as follows. First, the coupling device sends in all packets, including payload packets and signaling packets, into a memory subsystem and requests all the packets to be ordered. As a result, an ordering controller (for example, a PCI bus controller (PBC)) will ensure that each individual packet (including each of the payload packets) will be ordered. The first implementation results in unnecessary ordering and a significant throughput bottleneck. Second, the coupling device sends in all payload packets which are unordered and waits until all of the payload packets are complete; then, the coupling device sends in signaling packets. The second implementation has better throughput than the first implementation. The second implementation is a chosen protocol for IBM z13®.